Multiplicative stereophonic sound signalling system



Jan. 26, 1965 F. R. Hour ETAL 3,167,614

MULTIPLICATIVE STEREOPHONIC SOUND SIGNALLING SYSTEM Filed March 16, 1959 4 Sheets-Sheet 1 TACK vms FR :1S RHDLT Jari. 26, 1965 F. R. Hour E'rAl. 3,167,614

MULTIPLICATIVE STEREOPHONIC SOUND SIGNALLING SYSTEM Filed March 16, 1959 4 Sheets-Sheet 2 fear/lm) aff INVENTORJ TACK Avms FRANCIS H. HDL-r Jan. 26, 1965 R. HoLT ETAL. 3,167,614

MULTIPLICATIVE STEREOPHONIC SOUND SIGNALLING SYSTEM Filed March 16, 1959 4 Sheets-Sheet 3 mn W TSUM n mm.- M MVR i Am mm Am T r Jan. 26, 1965 F. R. HOLT ETAL. 3,157,614

MULTIPLICATIVE sTEREoPHoNIc soUND SIGNALLING SYSTEM Filed March 16, 1959 4 Sheets-Sheet 4 United States Patent O 3,167,614 MULTIPLICA'IIVE STEREOPHONIC SUND SEGNALLING SYSTEM Francis R. Holt, Willow Grove, Pa., and .lack Avins, New

York, N.Y., assignors to Radio Corporation of America, a corporation of Delaware Filed Mar. 16, 1959, Ser. No. 799,680 18 Claims. (Cl. 179-15) The present invention relates to new and improved apparatus for the transmission and reception of sterophonic sound signal information and, more particularly, to such apparatus capable of operation in compatibility with existing amplitude modulation (AM) radio broadcasts and receivers and AM bandwidth standards.

In view of the desirable feeling of presence afforded by stereophonic sound reproduction, there have been many proposals for the radio transmission and reception of stereophonic sound signals. These proposals involve, basically, two spaced microphones or other sources of stereophonically related signals (which may be designated the A and B signals, respectively, or, as another example, the sum and difference signals (A+B) and (A-B)), which stereophonically related signals are transmitted via a radio link to a receiver which translates the signals and derives the original A and B signals therefrom for respective application to separate sound reproducers.

One known form of stereophonic signal transmission system which has been employed commercially involves the transmission of one stereophonic signal via an ordinary AM broadcast transmitter and the transmission of the stereophonically related signal via a standard frequency modulated (FM) transmitter. These separately transmitted singals are then respectively intercepted, translated and reproduced by separate AM and FM radio receivers. Thus, stereophonic reproduction in accordance with such a system requires that the receiving location be equipped with two receivers, since neither an AM receiver nor an FM receiver by itself is capable of receiving and reproducing both of the stereophonically related signals. Such a system is, moreover, undesirable since it is not economical of channel bandwidth use.

In View of the foregoing, there have been many proposals in the past of compatible AM stereophonic signal transmission systems. The term compatible, as employed herein, means a system which permits a special receiver to pick up, translate and reproduce the twochannel stereophonic program from the transmitter, while the transmitted signal may also be received and translated by an ordinary AM receiver as a faithful monophonic sound reproduction. Further requirements of such a system are that the total transmitted signal should not exceed the bandwidth limitations imposed on standard AM broadcast transmissions, and that the stereophonic receiver also be capable of translating conventional monophonic AM broadcast signals.

General Prior art compatible AM stero systems may be classified, according to their signal malte-up, as additive or multiplicative By additive is meant a stereo signal which may b'e viewed a's comprising a carrier having inphase sidebands produced by amplitude modulation with one stereo signal, to which are added quadrature sidebands representative of a signal stereophonically related to that which produces the in-phase sidebands. By multiplicative is meant a system in which one stereo signal modulates a carrier wave to produce a carrier with sidebands which are then themselves modulated with a signal stereophonically related to the rst signal.

The present invention is of the multiplicative type. In order that the present invention may be better understood, however, a brief analysis of additive stereophonic systems ICC will first be made. One example of a prior art additive system is that in which two stereophonically related signals (e.g., A and B signals) are transmitted as amplitude modulation on the respective upper and lower sidebands of a carrier wave. This system may be termed an independent sideband system. In another form of prior art additive system referred to herein as the quadrature sideband system, the carrier wave is separated into two components (e.g., degrees displaced from each other), and these two carrier components are respectively amplitude modulated by the stereophonically related signals (eg, A and B). The modulated carriers and sidebands are then combined and transmitted. It has been shown that the resulting transmitted signal is equivalent to one in which a signal proportional to the sum of the two channels (i.e., A+B) is employed to amplitude modulate one phase of a carrier Wave while the channel-difference signal (A-B) is employed to amplitude modulate, with carrier-suppression, a quadrature phase of the same carrier wave.

lt may be noted that, in each of the foregoing forms of additive stereo systems, the resultant or transmitted signal may be represented as a carrier vector having in-phase sidebands representative of the (A+B) information, to which are added quadrature sidebands representative of the (A +B) information. Such a signal requires, for detection and separation of the A and B signals, rather complex receiver circuitry including, for example, means for precisely reconstructing or exalting the original carrier wave and circuits responsive thereto for synchronously detecting the received signal. Such systems are, therefore, not susceptible of use with simple, low cost stereophonic receivers.

An example of a prior art multiplicative system is one in which a carrier wave is first phase modulated by one stereophonic signal (e.g., the (A+B) signal) and the phase-modulated wave is then amplitude modulated by the other stereophonic signal (A +B). It may be noted that such a signal differs from that produced by an additive system in that its phase-modulation sidebands are themselves amplitude-modulated. Prior art multiplicative systems of this type require the use of receivers having precise synchronous detection circuits with the attendant circuitry for reconstructing the original carrier wave with great phase precision. Such a system is disclosed in British Patent No. 540,185 (1941)` It is, therefore, an object of the present invention to provide a new and improved compatible AM stereophonic sound signal system capable of operation within standard AM broadcast bandwidth limitations.

Another object of the present invention is to provide a new and improved stereophonic signal transmitter capable of producing and transmitting within the standard AM broadcast channel band a stereophonic signal which may be received and reproduced stereophonically in a stereophonic receiver or monophonically by a standard AM broadcast receiver.

In accordance with the present invention, there is provided a novel stereophonic sound signalling system of the multiplicative type, wherein a carrier wave is subjected to a special form of angular modulation in response to one stereo signal (eg, A-B) and then amplitude modulated with the corresponding stereophonically related sound signal (e.g., A+B). This special form of angular modulation may be viewed as combining, on a modulation frequency selective basis, both frequency modulation, for the lower portion of the audio frequency range, and phase modulation for the remainder of the audio range. Thus, the index of the angular modulation is generally inversely proportional to the modulating audio frequency 4below the mid-range frequencies (e.g.,

below 500 c.p.s.) and, for the remainder of the audio frequency range, the index is more nearly constant.

Viewed in another manner, the angular modulation of the carrier wave with one of the stereo signals, as described supra, is a special form of frequency modulation, differing in two main respects from conventional forms offrequency modulation: First, the system deviation, (i.e., the maximum carrier frequency deviation for low modulating frequencies), is extremely small, being a small fraction of the maximum audio frequency and, second, the higher audio frequencies are greatly preemphasized, in excess of 75 microseconds, prior to frequency modulating the carrier wave.

The doubly-modulated carrier wave is then transmitted Within the normal AM broadcast bandwidth limitations and may be translated by a novel receiver having a common channel from which the received signal is applied to two stereo channels, one channel including an AM signal detector and the other channel including an FM signal detector, with suitable translating means for applying the separated stereo A and B signals to respective sound reproducers.

It is important to note that a multiplicative signal requires twice the bandwidth of a standard AM channel, since each sideband produced by one modulating signal is itself modulated by the other signal, thus acquiring additional sidebands. For example, if the (A +B) modulation extends to 7.5 kc. and the (A -B) modulation also extends to 7.5 kc., the multiplicative modulation also sidebands which occupy a 30 kc. channel. It is a feature of the present invention, however, that the components of the instant multiplicative signal which extend beyond the normal $7.5 kc. bandwidth need not be transmitted, thus making the transmitted signal compatible with the standard AM bandwidth requirements, without sacrificing signal to noise ratio and without introducing undesirable inter-modulation distortion. This is made possible by the fact that the angular modulation is of such type that the phase deviation of the modulated carrier wave is inversely proportional to audio modulating frequency for the lower audio frequency range (e,g., below 500 c.p.s.) and in which the phase deviation is relatively constant for mid-range and higher audio frequencies.

Additional objects and advantages of the present invention will become apparent to those skilled in the art from a study of the following detailed description of the accompanying drawing, in which:

FIG. l illustrates several vector diagrams to be decribed;

FIG. 2 is a block diagram of a transmitter in accordance with the invention;

FIG. 3 is a block diagram of a receiver in accordance with one form of the present invention;

FIG. 4 illustrates certain receiver circuit characteristics to be described;

FIG. 5 is a block and schematic circuit diagram of a receiver in accordance with another form of the invention;

FIG. 6 is a schematic circuit diagram of a transmitter;

FIG. 7 is a schematic circuitdiagram of a receiver; and

FIG. 8 illustrates an alternative form of transmitter.

Overall description In accordance with the invention, a single carrier wave of suitable radio frequency may be rst modulated in either angle (as dened above) or amplitude by one stereophonic signal and, thus modulated, be then amplitudeor angle-modulated, respectively, with the stereophonically related signal. Thus, the carrier wave may rst be amplitude modulated with one stereophonic signal and then angle modulated with the stereophonically related signal. In the system to be described herein by way of illustration, however, the carier wave is rst anglemodulated in accordance with one stereophonic signal and the angle-modulated carrier wave is amplitude modulated with the other, stereophonically related, signal.

The compatibility of the present stereophonic signalling system with standard AM broadcast results, inter alia, from the fact that the angle modulation of the carrier wave (e.g., by the stereo difference signal (A-B)) is extremely narrow band, whereby the maximum deviation of the carrier frequency for low frequency modulating signals corresponds to only a small fraction of the system bandwidth (being, for example, 500 c.p.s. maximum deviation). This is in contrast to conventional narrow band or wideband FM in which the ratio of frequency deviation to channel bandwidth is of the order of unity. Viewed in another manner, the modulation index of the angle modulation in accordance with this invention (i.e., the ratio of carrier frequency deviation to maximum audio frequency to be transmitted) is a small fraction, of the order of 1/s to 1/20, by way of illustration. In contrast thereto, it may be noted that conventional wideband FM has a modulation index ofthe order of 5, while so-called narrow band FM has a corresponding index of the order of unity. The foregoing figures for modulation indices are given without regard to preemphasis of the audio signal.

It has already been pointed out, however, that the system of the present invention is further characterized in that it employs a substantially high degree of preemphasis, which is made possible by the very low frequency deviation of the system. Thus, for example, while standard wideband FM involves preemphasis of the order of 75 microseconds, it has been found that the extremely low system deviation involved in the present system permits a much greater order of preemphasis, as, for example, l5() nsec. to 300 aseo., or more.

The manner in which the small deviation for lower frequency modulating signals and the correspondingly large preemphasis of higher frequency modulating signals complement each other will now be described. The signal-to-noise ratio of an FM signal decreases as the frequency deviation is decreased. Additionally, crosstalk from the AM (A+B) sidebands into the (A-B) sidebands increases for higher modulating frequencies. The low carrier-frequency deviation is important from the standpoint of compatibility with existing AM broadcast receivers. The apparent disadvantages of this low deviation are effectively overcome by the large degree of high frequency preemphasis applied to the signal at the transmitter and the corespondingly high degree of high frequency deemphasis performed at the receiver.

It should further be noted that for low audio frequencies, as, for example, those below 500 cycles per second, the transmitted signal has more energy in its sidebands which represent the (A-B) information than is possible in any of the known additive systems. The reason for this is that, in frequency modulation, the phase deviation between the unmodulated carrier and the frequency modulated carrier is inversely proportional to the modulating audio frequency. Thus, as is shown in FIG. 1(a) the phase deviation 01 of the modulated carrier vector F,n with respect to the unmodulated carrier vector FC is of the order of 211- radians, or more, such that the frequency modulation information may be detected by simple FM detector circuits. This is to be contrasted with other systems in which the quadrature sideband components contain a relatively small amount of energy for low signal frequencies, with correspondingly smaller detectable phase deviations which are, therefore, not susceptible of efficient detection without resort to relatively complex synchronous detection circuits.

At mid-range frequencies (e.g., around 500 c.p.s.), the rate of change of the phase angle 02 is sufficiently large, as is shown in FIG. l(b) to permit detection by FM detectors. For still higher audio signal frequencies. (e.g., 5000 c.p.s.) it should be noted that in the absence of preemphasis, the sideband energy is extremely low such that the phase angle 03 may be of the order of a tenth of a radian. Such a reduction in the energy of the (A -B) sideband components would normally have the effect of rendering difficult their detection with an FM detector, both from the standpoint of noise and that of cross-talk. That is, a small quadrature noise component would be comparable in amplitude to that of the desired quadrature sideband information. Moreover, any relatively small asymmetry of phase or amplitude in the transmission characteristics of the system would produce quadrature components of the A+B modulation comparable in amplitude to that of the desired quadrature information.

By reason of the substantial degree of high frequency preemphasis applied to the audio signal prior to the frequency modulation of the carrier wave, however, these undesirable effects are substantially eliminated. FIG. 1(0) shows the effect of preemphasizing high frequency portions of the signal. This effect is that of increasing the phase angle 03 to 04, such that the phase angle 04 as shown by the dotted line vectors is of the same order as that developed for mid-range audio frequencies, as shown in FIGURE 1(b). In this manner, both the signal-tonoise ratio and freedom from quadrature cross-talk are greatly improved.

Another advantage of the present invention is that there need be no loss of stereophonic information at low or high audio frequencies, as is the case with many prior art additive systems which, for example, transmit the (A-B) information only for signals above some lower limit, such as 300 c.p.s., and below some upper limit, such as 3500 c.p.s. Such limitations of the stereophonic frequency range are placed on some prior art systems in an effort to improve their compatibility and to facilitate the reconstruction of the carrier wave at the receiver, as required for the detection of the quadrature sideband information.

Nevertheless, if it is considered desirable to decrease the transmission of (A-B) at either the low or high frequency ends, -it is clear 'that the present invention is still directly applicable to such an arrangement.

Thus, it will be readily apparent that the stereophonic signal produced and utilized in accordance with the present invention permits maximum utilization of the channel bandwidth to provide improved compatible stereophonic transmission, which may be translated and reproduced by a simple and inexpensive form of receiver.

General description of transmitter FIG. 2 illustrates, by way of a block diagram, a transmitter arrangement suitable for operation in accordance with the present invention. The .transmitter comprises sources and 12 of stereophonically related signals. Thus, by way of illustration, the source 1f? supplies the signal from one pickup which may be designated the A signal, While the source 12 provides the corresponding B signal. The A and B signals are applied to a matrix circuit 14 which serves to add and subtract the A and B signals to provide at its output leads 16 and 18, respectively, the stereophonically related (A-l-B) and (A-B) signals.

The (A-B) signal is amplified in an amplifier circuit 20 and is preemphasized in the circuit 22 which may have a preemphasis characteristic of the order of 150 or more microseconds. The preemphasis (A -B) signal is applied to a frequency modulating circuit 26 which may, for example, comprise a carrier wave oscillator and reactance modulator for deviating the frequency of the carrier wave in accordance with the amplitude of the (A -B) signal. As has been stated, the low-signal frequency deviation is relatively low, being of the order of i500 to 1000 c.p.s. maximum deviation for low signal frequencies up to 500 c.p.s., for example. The angularly modulated carrier wave is applied to an amplitude modulator 2S.

The (A+B) signal is amplified by successive audio amplification stages 30 and 32 and is, in turn, suitably applied, as by a cathode follower circuit 34, to the amplitude modulator 23 wherein it serves to amplitude modulate the frequency modulated carrier wave.

The output of the amplitude modulator 28 is, therefore, a carrier wave having angular modulation sidebands corresponding to the (A -B) signal information, the carrier and the sidebands being themselves modulated in amplitude by the (A -i-B) information. The output signal of the modulator 28 may, if desired, be limited in bandwidth as by means of a bandpass filter 36 shown as having a 20 kc. passband characteristic, although it has been found that the normal AM transmitter characteristics are such that the inclusion of a separate bandpass filter may be unnecessary, since the out-of-band radiation may not be excessive. The multiplicative signal thus produced is transmitted over the air by a transmitter 38 and antenna 4f).

FIG. 2 further illustrates a block 42 designated pilot tone oscillator which may be selectively connected to the (A i?) channel by means of a switch S1. The pilot tone oscillator 42 may be, for example, designed to produce a continuous wave of relatively low frequency (e.g., 25 c.p.s.) or, alternatively, of relatively high frequency. The purpose of the pilot tone oscillator 42 will be described hereinafter.

General description of receiver A stereophonic signal receiver suitable for stereophonic reproduction of the signal supplied by the transmitter 33 (FIG. 2) is illustrated by way of a block diagram in FIG. 3. The receiver comprises an antenna 44, which intercepts the multiplicative signal, and conventional RF, mixer, local oscillator and IF amplifier stages 46, 43, 50 and 52, respectively.

An AM detector 54 receives the intermediate frequency wave from the amplifier 52 in a conventional manner and detects the envelope thereof which, as will be understood, represents the (A+B) signal employed in amplitude modulating the carrier wave at the transmitter. The (A+B) signal is amplified in an audio amplifier stage Se and is applied to one input terminal of a matrix circuit 5S.

As thus far described, the receiver of FIG. 3 may be a conventional AM broadcast receiver and may, in fact, comprise conventional AM receiver circuits, including an AGC signal-deriving circuit. The AGC circuit may be in the form of either a separate diode detector or a circuit directly responsive to the AM detector 54 for providing the normal automatic gain (volume) control voltage for application to the IF amplifier 52 and RF amplifier 46, in a well known manner.

FIG. 3 further illustrates, in accordance with the present invention, an FM channel connected to receive the intermediate frequency signal from the common IF amplifier 52 via a lead 60. The FM channel, the purpose of which is to recover the (A -B) information from the received carrier wave, comprises an FM detector circuit 62, with suitable limiting action to be described more fully hereinafter, and a deemphasis circuit 64. The circuit 64 has a deemphasis characteristic which complements the preemphasis characteristic described in connection with the transmitter of FIG. 2. Thus, the circuit 64 serves to remove the preemphasis of the (A -B) signal at audio frequencies, rendering it a substantial replica of the (A .B) signal applied to the preemphasis circuit 22 at the transmitter. The deemphasized (A -B) signal is amplified in an audio amplifier 68 and applied to a second terminal of the matrix circuit 58.

The matrix circuit may comprise any suitable addition and subtraction networks of either the passive or active variety, and serves to reconstruct the individual A and B signals which are applied, respectively, for audible reproduction by loudspeakers 70 and 72.

receiver in accordance with the invention.

The FM detector circuit 62 is indicated in the drawing as including a dynamic limiter. The reason for the employment of a dynamic limiter for the frequency modulated signal will be recognized from the following. The dynamic limiting and FM detection functions may be performed in a single circuit, such as a ratio detector.

An ordinary AM receiver with conventional AGC action, `such as that illustrated in FIG. 3, does not have a flat input-output characteristic. Rather, its characteristic is of gradual slope, such as is shown by the curve 74 in FIG. 4, which curve represents a plot of the AM detector output as a function of the RF input to the receiver, with normal AGC action.

Proper stereophonic reproduction requires that there be balance between the A and B signals applied to the loudspeakers '70 and 72, and, therefore, between the (A+B) and (A -B) signals from which the individual signals are reconstructed. Thus, it is desirable under normal conditions that the amplitude of the (A-B) signal track that of the (A+B) signal. Such tracking would not occur if the angularly modulated signal were limited in a static manner such that the amplitude of the detected signal were completely independent of amplitude variations of the frequency modulated signal, Thus, as is lshown in FIG. 4, the characteristic of a conventional static limiting device is a curve 7h having a sharp rising characteristic at low input levels, followed by a knee and a relatively ilat output-vs.input amplitude characteristic for the remainder of the range of input signal levels. It is apparent from FIG. 4 that the use of a static limiter by itself and without other compensation will result in lack of balance between the FM and AM channel signals.

Thus, the present invention includes means for insuring that the output level of the FM (or A-B) channel is in proportion to the strength of the IF signal, whereby the FM channel signal amplitude may track that of the AM signal amplitude.

Such tracking may be achieved, as stated, by the use of an FM detector having dynamic limiting characteristics. One such detector is the ratio detector which has a linear output-input characteristic which is modified by the normal AGC of the receiver to provide a characteristic matching curve 74 in FIG. 4. Thus, with dynamic limitingraction, the audio amplitude at the FM detector output will very in proportion to the amplitude of the intermediate frequency signal input.

It should be noted that a circuit such as the ratio detector includes a relatively large capacitor in its output network, so that it does not respond to amplitude liuctuations `at an audio rate.

Hence, the use of a dynamic FM detector circuit for performing the function of the block 62 results in the (A-B) detector output signals varying in the same manner as the (A+B) output of the AM detector, giving automatic tracking of the (A+B) signal by the (A-B) signal.

From the foregoing, it will be recognized that the receiver of FIG. 3 is capable of full stereophonic reproduction from the multiplicative signal described supra. Moreover, the receiver is capable of compatible reproduction of monophonic sound from a standard AM broadcast, since the AM detector channel responds to conventional monophonic AM signals as Well as it does to the AM component of the multiplicative signal. When operating upon a conventional monophonic AM broadcast signal, it may be desirable to disable the FM channel of the receiver, as by means of a switch S2 which effectively open-circuit that channel. With the FM channel thus disabled, the possibility of response of the channel to spurious phase or frequency modulations of the transmitted monophonic broadcast is precluded.

FIG. 5 illustrates another embodiment of a sterephonic In view of the similarity between the receiver of FIG. 5 and that of FIG. 3, corresponding parts will be designated by the same reference numerals.

The receiver of FIG. 5 includes the same AM receiver portions corresponding to the elements 44 through 56 described in connection with FIG. 3 and comprising the common and AM signal processing channels. The FM detector-limiter stage 82 in FIG. 5 differs, however, from the detector 62 of FIG. 3 in that the circuit 82 includes a static limiter having an output-input characteristic such as that shown by the curve 76 in FIG. 4. The output signal from the stage 82 is applied to a deemphasis circuit 84 and the deemphasized (A-B) signal is applied via a resistor d5 to an audio amplifier 88. The deemphasis circuit 84 has a deemphasizing characteristic which corresponds to the characteristic described in connection with FIG. 3. The amplified (A+B) and (A-B) signals are applied to a matrix circuit 58 which serves to reconstruct the A and B signals `for application to respective loudspeakers '70 and 72.

Since, as was described in connection with FIG. 4, a limiter circuit having a static characteristic (curve 76 in FIG. 4) will not serve to produce balancing between the (A+B) and (A -B) signals, an additional circuit is shown in FIG. 5 for affording automatic stereo balance between the (A+B) and (A -B) signals, or stated otherwise, to maintain a desired ratio between the (A -B) and (A+B) signals. This control circuit may comprise simply a triode 90 Whose anode 92. is coupled to the junction of the resistor 85 and the audio amplifier 88. The control electrode 94 of the triode 90 is connected adjustably to a potentiometer 96 which, in turn, is connected to provide across its terminals a negative-going voltage corresponding to the normal ACG voltage.

Thus, it will be seen that, with the tap set on potentiometer 96 for proper steady state balance of the (A +B) and (A +B) signals, which may be determined by the listener, the triode will receive on its control electrode a negative voltage which controls its conduction. If the strength of the incoming signal should, for example, decrease, such that the (A+B) signal amplitude would decrease, the voltage applied to the control electrode 94 would become less negative, reflecting the reduced AGC voltage applied to the common channel stages such as the RF amplifier 46 and IF amplifier 52. Such reduction of the negative voltage applied to the control electrode 94 would, in turn, result in an increase in conduction through the triode which would produce a greater shunting action to ground of the audio frequency (A -B) signal, and in such manner as to track the gain characteristics of the AM detector channel. Conversely, the control triode 90 serves to match the signal output characteristic of the (A+B) channel to that of the (A+B) channel with increasing signal strength.

While the inclusion of an automatic ratio control circuit such as that described is normally quite desirable, there are certain conditions under which it may not be desirable to maintain automatically a constant relation between the intermediate frequency input signals applied to the AM and FM detectors. For example, it may be preferable under weak signal conditions to eliminate the stereophonic effect in the interest of improving the signal-to noise ratio of the reproduced signal. That is, substantial AM suppression under weak signal conditions requires relatively expensive circuitry in the FM channel. Hence, in an inexpensive receiver having less than optimum AM suppression under weak signal conditions it may be desirable to disable the (A-B) channel of such a receiver under weak signal conditions to eliminate the cross-talk and distortion which might otherwise impair the reproduction of the (A +B) signal. Thus, the automatic stereo control circuit including the triode 90 may be so biased that when the IF signal input as indicated by the AGC voltage falls below a predetermined value, the tube 90 will conduct sufficiently to provide an effective short-circuit shunt path across the (A -B) channel.

Another condition under which disabling of the (A -B) channel is desirable exists during the reception of a standard monophonic AM signal, because of incidental angular modulation distortion which may be transmitted by the AM transmitter, as when it is over-modulated.

Such automatic disabling of the (A-B) channel during the reception of a monophonic broadcast signal may be accomplished in the following manner.

Means may be provided at the stereoph'onic broadcast transmitter for transmitting a signal identifying the broadcast as a stereophonic one. Such an identifying or flag signal may take the form of a continuous wave which is transmitted With the stereo broadcast signal, either as amplitude modulation or angular modulation of the can rier wave. FIGS. 2 and 5 illustrate an arrangement in which the pilot or identifying signal is a low frequency (e.g., 25 c.p.s.) signal transmitted as angular modulation of the carrier Wave. This identifying signal is provided by a suitable oscillator 42 whose output wave may be applied to the angular modulation channel. With the switch S1 closed, the pilot tone Will itself produce angular modulation of the carrier wave in the circuit 26 and will be transmitted as a part of the multiplicative signal.

In the receiver, the pilot modulation thus applied to the carrier Wave Will be detected by the FM detector 82 and may be selectively removed lfrom the (A-B) information, as at the output of the deemphasis circuit 84, by means of a suitable filter 100, which may be a sharply tuned filter at the frequency of the pilot modulation. The signal provided by the filter may then be applied to a rectifier circuit 102 which serves to produce a direct current voltage in response to the separated pilot modulation. The rectifier circuit is so polarized as to apply to the control electrode 104 of the control tube 165 a negative voltage during the reception of a stereo signal including the identifying pilot modulation. This negative voltage applied to the control electrode 1M effectively cuts off the tube 10.6, preventing it from shunting the signal appearing at the right hand end of the isolating resistor S5. Thus, the (A -B) signal is applied in the normal manner to the audio amplifier 88 for further application to the matrix circuit, as described.

During the reception of a monophonic AM broadcast lacking the identifying pilot modulation, there will be no signal applied to the rectifier circuit 102 by the filter 160, so that no cut-off bias is applied to the tube 106. In this case, the tube 106 will conduct sufiiciently to provide an effective short-circuit path across the (A-B) channel, thereby preventing any spurious signal components from being amplied and applied to the matrix circuit.

While the arrangement for automatically enabling the stereo channel of the receiver during reception of a stereo broadcast has been described in connection with angular modulation of the carrier wave with a low frequency pilot tone, it will be recognized that a pilot frequency Within the audible range may be employed, requiring the addition of selective trap circuitry in the (A -B) channel to remove the pilot signal from the path leading to the matrix and loudspeakers.

In accordance with another form which the invention may take, the identifying pilot modulation may be transmitted during a stereo broadcast by amplitude modulation of the carrier Wave with an inaudible frequency tone. An advantage of transmitting the pilot information by amplitude modulation is that the angular modulation channel of the receiver need not be activated during reception of a monophonic signal. Rather, the pilot tone may be detected in the AM detector channel and applied to cut-off the angular modulation channel in its entirety. Where transmission of the pilot tone by amplitude modulation is employed, the pilot modulation should be of relatively low frequency so that it is not audible on existing monophonic receivers, whereas when the pilot modulation is transmitted as part of the angular modulation signal, a simple trap circuit may, as has been menl@ tioned, be inserted in the (A-B) angular modulation channel to remove all traces of the pilot tone.

It will further be noted that the receiver of FIG. 5 includes a switch S2 which may be manually operated to disconnect the angular modulation (A-B) channel during reception of a monophonic signal.

Illustrative transmitter circuitry While, as may be noted from the block diagram of FG. 2, a standard AM broadcast transmitter may be readily modified to operate in accordance with the present invention, a complete transmitter system will be described herein in the interest of illustration. Before describing a specific transmitter arrangement, however, it should be noted that a standard AM broadcast transmitter may be simply modified by the substitution of an angular modulated oscillator for the usual crystal-controlled carrier Wave oscillator of the transmitter. The remainder of the necessary apparatus, including the matrixing circuit and angular modulating circuits including preemphasis networks, may be of any suitable variety for performing the functions described in connection with FIG. 2.

FIGURE 6 is a schematic diagram of a complete transmitter, wherein signals from the two stereophonically related sound sources are applied to the input terminals and 112. As shown, these input signals may be the -A and -B signals, respectively. The -A signal is amplified and inverted in polarity by the lefthand section of the dual triode preamplifier stage 114 and appears as the A signal in the anode load circuit of the tube. Similarly, the -B signal is amplified and inverted by the righthand section of the preamplifier 114 and appears as the B signal across the potentiometer in the anode circuit of that section of the tube. The A and B signals are matrixed to produce the (A +B) and (A-B) signals in the following manner.

The A signal is applied to the control electrode of a phase splitter tube 116 which provides the -A and A signals at its anode and cathode output terminals 118 and 12). The B signal is amplified and inverted in polarity by an amplifier 122 and appears as the -B signal at the anode output terminal 124. The -A signal from the terminal 118 is coupled via a capacitor 126 to the control electrode 12S of an adder tube 130, While the -B signal from the terminal 124 is coupled by means of a capacitor 132 to the control electrode 12S. Thus, the signal appearing at the electrode 128 is the signal (-A-B), so that the output signal of the amplifier 131B at terminal 134 is the sum signal (A -l-B).

The -B signal from the terminal 124 and the A signal from the terminal are also coupled, respectively, to the control electrode 136 of an adder tube 138, so that the input signal to that tube is the (A -B) signal.

Included in the anode circuit of the tube 133 is a series inductance-resistance circuit comprising the inductor 141i and resistor 142 which, together, form a preemphasis network which, for the values shown, has a preemphasizing characteristic of the order of 200 microseconds. Hence, the (A-B) signal is preemphasized by the circuit associated with the tube 138 and appears as a preemphasized (A-B) signal at the terminal 144.

The \-(A-B) signal is adjustably applied via a balancing volume control potentiometer 145 to the control electrode of the pentode section of the reactance modulator tube 14S. The pentode section of the tube 148 is the reactance tube portion of the modulator circuit, while the carrier wave oscillator circuit is associated with the triode section of the tube. It will thus be understood that the preemphasized (A -B) signal serves to frequency modulate the carrier Wave produced by the oscillator circuit, deviating its frequency as described above. Specifically, the FM modulator shown in connection with tube 148 is so designed as to produce a maximum deviation for 10W- frequency modulating signals of the order of 500 c.p.s.

The angular modulated carrier Wave is applied via coupling capacitors to the control electrode 150 of an amplitude modulator tube 152.

Insofar as the remainder of the apparatus of FIG. 6 is concerned, it will be noted that the (A +B) signal from the terminal 134 is amplified by an audio amplifier stage 154 to which it is applied via a balance or volume control potentiometer 156. The signal thus appears as the -(A+B) signal at the anode terminal 158 and is applied to the control electrode 160 of a cathode follower stage 162. The (A +B) signal appearing at the cathode terminal 164 serves to amplitude modulate the angular modulated carrier wave appearing in the anode circuit of the amplitude modulator tube 152.

The resultant signal appearing across the secondary winding of the transformer T1 thus comprises the multiplicative angular and amplitude-modulated carrier wave which is in acordance with the description given above. The modulated carrier wave is applied by a potentiometer 166 to an output terminal 168 from which it may be applied to suitable additional amplification and band-limiting circuits before transmission.

Where a stereo identifying signal such as the FM pilot tone described earlier is to be transmitted, it may be applied to the terminal 170 at the input of the frequency modulation stage.

It may also be noted that the apparatus of FIG. 6 includes a switch S3 which may be employed to select the mode of operation of the transmitter. In the position shown in the drawing, the transmitter apparatus is conditioned for stereophonic signal transmission in accordance with the invention. When the rotor of the switch S3 is moved to the lowermost terminal, it disables the (A-B) channel, so that only the (A +B) AM channel is operative. Conversely, when the rotor of the switch S3 is in contact with the uppermost terminal, the (A +B) channel is rendered inoperative, so that only the (A -B) channel is effective. The last-named position of the switch may be used, for example, where it is desired to transmit only the (A -B) signal for testing purposes.

Description of illustrative receiver circuits FIG. 7 is a circuit diagram showing in its entirety the AM and FM channels of a stereophonic signal receiver in accordance with the invention. Since the RF, heterodyning mixer and oscillator, and first IF stages of the receiver may be of any suitable form, the circuit of FIG. 7

includes only that portion of the receiver which includes and follows the final IF amplifier stage.

The intermediate frequency version of the received stereophonic signal thus appears at the input terminal 176 and is amplified in the circuit of the amplifier tube 178, appearing across the tuned circuit 180 which is resonant at the intermediate frequency. Connected across the tuned circuit 160 in signal-receiving relation therewith is an AM detector diode 182 which serves in a conventional manner to detect the amplitude modulation of the received carrier Wave. The detected AM (A+B) signal is thus presented across the potentiometer 184 which serves as a manual stereo control which may be adjusted to set a desired relation between the (A+B) and (A+B) signals. The (A+B) signal is applied from the potentiometer 184 via a lead 136 to the control electrode of an audio amplier stage 188.

Also appearing across the tuned circuit 160, at the terminal 19t), is the intermediate frequency version of the received multiplicative wave for application to the FM channel of the receiver. It will be noted that, in the interest of reducing undesirable phase modulation which might result from the action of the AM detector 182 connected across the tuned circuit 180, the circuit 180 is highly damped by a shunt resistor 192.

The signal from terminal 1% is limited by successive cathode-coupled limiter stages 194 and 196 which are of conventional form and which are of the static-limiting type described earlier in connection with FIG. 5. The

limited intermediate frequency signal is coupledfr'om the second limiter 196 by rrieans of 'a transformer T2 to the FM detector circuit 198 which provides the detected (A+B) signal 'at its output terminal 261i. This signal is, as explained in connection with FIG. 6, preemphasiied by an amount of the order of 200 nsec., and must, therefore, be deemphasized before it may be combined with the (A+B) signal foi' matrixing. The deernphasizing function is performed by the series-shunt resistor-capacitor combination 202, 204 which provides the required degree of deernphasis, supplying the deeinphasized signal to the input terminal 266 of a phase splitter tube 208. The phase splitter tube 20S provides at its output leads 212 and 216, respectively, the (A-B) and (A-B) signals which are, in turn, applied to the control electrodes 214 and 216 of the dual triode matrix stage 218.

Simultaneously, the detected (A+B) signal is amplified and inverted by the tube 18S and is applied as a -(A+B) signal to the electrodes 214 and 216. The tube section associated with the electrode 214 thus adds the (A-B) and -(A+B) signals and provides at its output lead 220 the signal corresponding to the A signal.

Similarly, the tube section associated with the control electrode 216 adds the signals to provide at its output lead 222 a signal corresponding to the B signal.

The separated A and B signals appearing at the terminals 224 and 226, respectively, corresponding to the A and B signals supplied to the transmitter circuitry of FIG. 6 and may be translated to suitable sound reproducing devices (not shown) for stereophonic sound reproduction.

It is an advantage of the present invention that the multiplicative stereophonic signal may be detected in a simple receiver in which tuning is relatively non-critical particularly when compared with the precise and critical tuning required of synchronous detection receivers of the type employed with additive signals. It will been seen, however, that automatic tuning control or AFC may be readily provided with the receiver of FIG. 7. For example, the voltage appearing at the terminal 266 is suitable for application, after low-pass filtering, to a reactance device, since the FM detector 198 is a balanced detector so that its output at the terminal 206 is indicative of any detuning of the receiver. The reactance device may be of any known form, such as a variable capacitance diode connected across the receiver local oscillator circuit (not shown in FIG. 7). By this expedient, and at relatively low cost, precise tuning control may be provided. Alternatively, or in addition, the voltage at the-terminal 206 may be applied, after filtering, to a tuning eye indicator.

Further in connection with FIG. 7, it should be noted that there is connected to the terminal 230 a ratio ((AB)/(A+B)) control tube 232 which corresponds in function and operation to the ratio control tube described in connection with FIG. 5. The tube 232 in FIG. 7 is connected in shunt with the output of the limiter circuit, receiving its anode energizing voltage from a B+ terminal 234, via a decoupling network comprising a resistor 236 and shunt capacitor 238. A control voltage for the conduction of the tube 232 is derived from the terminal 191 at the output of the AM detector, corresponding to the normal AGC voltage, indicated at the terminal 240. The operation of the automatic ratio control tube 232 will be readily understood from the earlier description of the circuit 9i) shown in FIG. 5.

It will also be understood that a suitable pilot modulation filter, rectifier circuit and stereophonic channel enabling device may be connected in the FM channel of the receiver of FIG. 7, as at the terminal 236, in the same manner as that described in connection with the elements 106, 162 and 106 of FIG. 5.

While the present invention has been illustrated herein as involving simple preemphasizing and deemphasizing (of a high order), it should be recognized that more compiex preemphasis and deemphasis characteristics may be used. For example, if it is desired to provide a more rapid slope of preemphasis and deemphasis than is provided by the single R-L and R-C time constants, a multiple timeconstant may be used to afford increased preemphasis and deemphasis at the higher frequencies. Such a characteristic may be provided at the transmitter, for example, by applying the output signal of the preemphasis network to a pentode amplifier, or other constant current device, the output of which is passed through a second series resistor-inductor preemphasizing network. At the receiver, the deemphasizing network may be matched to the double time-constant preemphasis by adding a second R-C network to that shown in FIG. 7 (elements 202 and 204). Such double time-constant preemphasis and deemphasis will have the effect of further increasing the signal-to-noise ratio and of reducing AM to FM cross-talk at the higher signal frequencies.

Alternative transmitter embodiment It is important to note that while the transmitter portion `of the present system has been described, apparatuswise, as including a frequency modulator having low system deviation, with means for greatly preemphasizing the higher audio frequencies, the resultant signal is, as stated, one which partakes of the nature of both narrow band frequency modulation for lower audio frequencies and phase modulation fo-r the remainder of the audio range.

This novel form of angular modulation, as defined above, may be performed alternatively by a phase modulator, in which event the audio signal is irst preemphasized in reverse sense. That is, prior to application to the phase modulator, the amplitude of the audio signal for lower frequencies (e.g., below 500 c.p.s.) is boosted as `an inverse function of frequency, such that its amplitude is increased as its frequency decreases.

FIG. 8 shows a transmitter arrangement in accordance with this form of the invention. In FIG. 8, there are illustrated sources of (A-B) and (A+B) signals 260 and 262. The (A-B) signal is applied to an audio signal correcting network 264 which boosts or preemphasizes the lower audio frequencies (e.g., those below 500 c pts.) such that audio frequencies of 50 c.p.s. are boosted in amplitude 10:1 with respect to the 500 c.p.s. signal. The network 264 is further characterized in that its amplitude-vs.frequency response is relatively ilat for audio frequencies .above 500 c.p.s.

The corrected (A-B) signal is applied to a phase modulator 266 which also receives a carrier wave of a frequency f, from a source 268 which may be a crystal oscillator. The modulator 266 varies the phase of the carrier wave in accordance with well known phasemodulation principles. The phase-modulated carrier wave is frequency multiplied by a factor N, by a frequency multiplier 269 and is then applied to an amplitude modulator 270 where it is modulated in amplitude as a function of the (A +B) signal from source 262.

The resultant angleand amplitude-modulated carrier wave is then transmitted via the transmitter 272 and antenna 274.

The transmitter 272 may include suitable filtering means to limit the 'bandwidth of the signal supplied to the antennaV 274 to approximately twice the frequency of the highest modulating signal.

This transmitted signal may be received and properly translated by a receiver such as that described in connection with FIGS. 3, 5 and 7.

As used in the specification and the appended claims, the term angular modulation means the special form of angular modulator described above, whereby a carrier wave is angularly modulated in such manner that, for lower modulating signal frequencies the index of modulation is generally inversely proportional to the modulating signal frequency while for mid-range and higher frequencies the index of modulation is relatively constant.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent is:

1. A stereophonic sound signalling system for transmission land reception within a given transmission channel bandwidth, said system comprising: a source of first and second stereophonically related signals; a source of a carrier wave; means coupled to said signal source .and said carrier wave source for angle modulating and amplitude modulating a carrier wave with said first and second signals, respectively, to produce a multiplicatively modulated carrier wave having both angle modulation and amplitude modulation sidebands wherein the maximum frequency deviation of said carrier wave, in respouse to maximum amplitude modulating signals of frequencies up to the order of said maximum deviation, is a small fraction of such channel bandwidth, said means for angle modulating said carrier wave including means for preemphasizing the higher modulating frequencies of said rst signal by a predetermined amount; means for transmitting said multiplicatively modulated carrier wave; and a receiver for said transmitted wave, said receiver comprising a common amplifying channel for the reception and amplification of said multiplicative wave, a iirst stereo channel coupled to said common channel and including an amplitude modulation detector for detecting the amplitude modulation of said received wave to reproduce said second signal, a second stereo channel coupled to said common channel and including a frequency modulation detector for recovering the angle modulation component from said received wave and means for deemphasiZ-ing said recovered angle modulation component by an amount corresponding to the amount of preemphasis applied to said first signal, a sound reproducing system, and means for applying said recovered second signal and said deemphasized first signal to .said sound reproducing system.

2. A stereophonic sound signalling system for transmission and reception within a transmission channel bandwidth of substantially twice the highest modulating signal frequency, said system comprising: a source of first and second stereophonically related signals; a source of a carrier wave; means coupled to said signal source and said carrier wave source for angle modulating and amp'litude modulating a carrier wave with said irst and second signals, respectively, to produce a multiplicatively modulated carrier wave having both angle modulation and amplitude modulation sidebands wherein the maximum frequency deviation of said carrier wave, in response to maximum .amplitude modulating signals of frequencies up to the ord-er of said maximum deviation, is a small fraction of said channel bandwidth, said means for angle modulating said carrier wave including means for preemphasizing said first signal by an amount in substantial excess of 75 microseconds; means for transmitting said multiplicatively modulated carrier; and a receiver for said transmitted wave, said receiver comprising means including an envelope detector for detecting the amplitude modulation of said received wave to reproduce said second signal, means coupled to said common channel and including a frequency modulation detector `for recovering the angle modulation component from said received wave and means for deemphasizing said recovered angle modulation component by an amount corresponding to t-he amount of preemphasis applied to said first signal, a sound reproducing system, and means for applying said recovered second signal and said deemphasized first signal to said sound reproducing system.

3. A stereophonic sound signalling system for transmission within a given transmission channel bandwidth, said system comprising: a source o-f first and second stereophonically related signals; a source of a carrier wave; means coupled to said signal source and said carnier wave source for angle modulating and amplitude modulating a carrier wave with said i'irst and second signals, respectively, to produce a multiplicatively modulated carrier wave having both .angle modulation and amplitude modulation sidebands wherein the maximum frequency deviation of said carrier wave, in response to maximum amplitude modulating signals of frequencies up to the order of said maximum deviation, is a small fraction of said channel bandwidth; a source of identifying signal; means lfor transmitting said multiplicatively modulated carr-ier and said identifying signal; and a receiver for said transmitted wave, said receiver comprising a common amplifying channel for the reception and amplifiicatio-n of said wave, a first stereo channel coupled to said common channel and includingan einvelope detector for detecting the amplitude modulation of said received wave to reproduce said second signal, a second stereo channel coupled to said common channel and includ-ing a frequency modulation detector for recovering the angle modulation component from said received wave; a sound reproducing system, means for applying said recovered second signal and said rst signal to said sound reproducing system, and means resp-onsive to said identifying signal for effectively controlling the operation of said second stereo channel.

4. A stereophonic sound signalling system for transmission within a given transmission channel bandwidth, said system comprising: a source of first and second stereophonically related signals; a source of a carrier wave; means coupled to said signal source and said carrier wave source for angle modulating and amplitude modulating a carrier wave with said first and second signals, respectively, to produce a multiplicatively modulated carrier wave having both angle modulation and amplitude modulation sidebands wherein the maximum frequency deviation of said carrier wave, in response to maximum amplitude modulating signals of frequencies of up to the order of said maximum deviation, is a small fraction of such channel b-andwidth, said means for angle modulating said carrier wave including means for preemphasizing said first signal by a predetermined amount.

5. A stereophonic sound signalling system for transmission within a given transmission channel bandwidth, said system comprising: a source of first and second stereophonicarlly related signals; a source of a carrier wave; means coupled to said signal source and said carrier wave source for angle modulating and amplitude modulating a carrier wave with said first and second signals, respectively, to produce a multiplicatively modulated carrier wave having both angle modulation .and amplitude modulation sidebands wherein the max-imum frequency deviation of said carrier wave, in response to maximum amplitude modulating signals of frequencies up to the order of said maximum deviation, is a small fraction of such channel bandwidth, said means for frequency modulating said carrier wave including means for preemphasizing said first signal by an amount in substantial excess of 75 microseconds.

6. A soun-d signalling system for transmission of a range of audio frequency signals within a transmission channel bandwidth of substantially twice the highest modulating signal frequency, said system comprising: a source of first and second signals; a source of a carrier wave; means coupled to said signal source and said carrier wave source for angle modulating and ampl-itude modulating a carrier wave with said first and second signals, respectively, to produce a multiplicatively modulated carrier wave having both angle modulation and amplitude modulation sideb-ands wherein the maximum frequency deviation of said carrier wave lfor maximum amplitude modulating frequencies up to the order of said maximum deviation, is a small fraction of such audio frequency range.

7. A stereophonic sound signal receiver for a multiplicatively modulated carrier wave angle modulated by :a first stereophonic signal and .amplitude modulated -by a stereophonically related second signal, said receiver Cil comprising: .a co-mrnon-ampl-ifyingchannel for the reception and amplification of said wave, means coupled to said common channel and including an envelope detector for detecting the amplitude modulation of said received wave to reproduce said second signal and to provide .an AGC signal indicative of the strength of said carrier wave, means coupled to said common channel including a frequency modulation detector for recovering the angle modul-ation component from said received wave, a sound reproducing system, means for applying said recovered second signal and said iirst signal to said sound reproducing system, and means responsive to the strength of said received wave for controlling the relative amplitudes of output of said detectors.

8. A .stereophonic sound signal receiver for a multiplicatively modulated carrier wave angle modulated by a first stereophonic signal and amplitude modulated by a stereophonically rel-ated second signal, said receiver comprising: a common amplifying channel for the reception and amplitfication of said wave, a first stereo channel coupled to said common channel and includv ing an envelope detector for detecting the Aamplitude modulation of said received wave to reproduce said second signal and for providing an AGC signal for controlling the gain of said common channel, a second stereo channel coupled to said common channel and including a limiter circuit and a frequency modulation detector for recovering the angle modulation component from said received wave, and means responsive to said AGC signal for controlling the output-input characteristic of said second channel to maintain a predetermined balance characteristic between said first and second channels.

9. A stereophonic sound signal-ling system for transmission within a given transmission channel bandwidth, said system comprising: a source of an (A+B) signal and a stereophonically related signal; a source of a carrier wave; means coupled to said signal source and said carrier wave source for angle modulating a carrier wave with said stereophonically related signal and amplitude modulating said carrier wave with said (A+B) signal to produce a multiplicatively modulated carrier wave having both angle modulation and amplitude modulation sidebands wherein the maximum frequency deviation of said carrier wave, in response to maximum arnplitude modulating signals of frequencies up to the order of said maximum deviation, is a small fraction of such channel bandwidth, said means for angle modulating said carrier wave including means for preemphasizing said stereophonically related signal by an amount in excess of microseconds; and means for transmitting said multiplicatively modulated carrier.

l0. A stereophonic sound signalling system for transmission within a given transmission channel band-Width, said system comprising: a source of first and second stereophonically related (A-B) and (A -l-B) signals; a source of carrier wave; means coupled to said signal source and said carrier wave source for angle modulating said carrier wave with said (A -B) signal and amplitude modulating said carrier wave with said (A -l-B) signal, to produce a multiplicatively modulated carrier wave having both angle modulation and amplitude modulation sidebands wherein the maximum frequency deviation of said carrier wave for maximum amplitude (A -B) modulating signals of frequencies up to the order of said maximum deviation is a small fraction of such channel bandwidth, said means for angle modulating said carrier wave including means for preemphasizing said (A -B) signal by an amount in sub'-- stantial excess of 75 microseconds; and means for transmitting said multiplicatively modulated carrier.

ll. A stereophonic sound signal receiver for a multiplicatively modulated carrier Wave angle modulated by a first stereophonic signal and amplitude modulated by a stereophonically related second signal and a signal identifying said wave as including such angle modulation, said receiver comprising; a common amplifying channel for 17 the reception and amplication of said wave, a first stereo channel coupled to said common channel and including an envelope detector for detecting the amplitude modulation of said received wave to reproduce said second signal, a second stereo channel coupled to said common channel and including a frequency modulation detector for recovering the angle modulation component from said received wave, means for detecting the presence of such identifying signal and responsive thereto for disabling said second stereo channel during reception of a wave lacking said identifying signal and for enabling said second stereo channel during the reception of a wave including said identifying signal.

12. A stereophonic sound signalling system for transmitting and receiving stereo information contained in a band of audio frequencies within a transmission channel having a bandwidth of the order of twice said audio frequency band, said system comprising a source of first and second stereophonically related signals, a source of carrier waves, means for angle modulating said carrier waves and amplitude modulating said waves with said first and sccond stereophonic signals, respectively, said means for angle modulating said carrier wave being arranged so that, for relatively low frequencies of said first signal, the maximum frequency deviation of said carrier wave, in response to maximum amplitude modulating signals of frequencies of up to the order of said maximum deviation, is a small fraction of said bandwidth and, for higher frequencies of said first signal the modulation index is relatively constant; means for transmitting said angle modulated and amplitude modulated waves through a channel of bandwidth of the order of twice said audio frequency band, a receiver for translating said transmitted waves including means for detecting the envelope of said waves to reproduce said second signal and means including a frequency modulation detector and deemphasis circuit for detecting the angular modulation of said wave and reproducing said first signal.

13. In a stereo receiver for the reception of a carrier wave modulated respectively in angle and amplitude by a pair of stereo-related signals, a pair of demodulator stages, means for impressing waves derived from said received waves upon the stages, amplitude detection means in one of said stages for recovering the stereo-related signal that amplitude modulates the carrier, angle detection means in the other stage for recovering the stereo-related signal that angle modulates the carrier, said second stage including means for substantially eliminating recovery of derived carrier amplitude variations occurring at audible frequencies but permitting reproduction of variations occurring at a sub-audible rate.

14. A stereophonic sound signalling system for transmitting and receiving stereo information contained in a band of audio frequencies within a transmission channel having a bandwidth of the order of twice said audio frequency band, said system comprising a source of first and second stereophonically related signals, a source of car rier waves, means for angle modulating said carrier waves and amplitude modulating said waves with said first and second stereophonic signals, respectively, said means for angle modulating said carrier waves comprising a phase modulator, an audio frequency correcting network for boosting the low frequency components of said first signal prior to application to said phase modulator, and a frequency multiplying circuit coupled to said phase modulator for frequency-multiplying the output of said phase modulator, so that, for reulatively low frequencies of said first signal, the maximum frequency deviation of said carrier wave, in response to maximum amplitude modulating signals of frequencies of up to the order of said maximum deviation, is a small fraction of said bandwidth and, for higher frequencies of said first signal the modulation index is relatively constant; means for transmitting said angle modulated and amplitude modulated waves through a channel of bandwidth of the order of twice said audio 18 frequency band, a receiver for translating said transmitted waves including means for detecting the envelope of said waves to reproduce said second signal and means including a frequency modulation detector and deemphasis circuit for detecting the angular modulation of said wave and reproducing said first signal.

15. A stereophonic radio signal receiving system comprising the combination of, means providing a first signal translating channel for one of a pair of stereophonic signals, means providing a second signal translating channel for the other of said pair of stereophonic signals, means for deriving a control voltage the amplitude of which is a function of the level of said one of said signals, controlvoltage-responsive means in one of said channels for controlling the output signal level therefrom, and means for applying said control voltage to said control Voltage-responsive means to aid in maintaining a predetermined relationship of balance between the signal output levels of said first and second signal translating channels.

16. A stereophonic radio signal receiving jsystern for stereophonic or monaural signals wherein said received stereophonic radio signals include modulation components corresponding to a pilot signal of predetermined frequency, comprising the combination of, means providing a first signal translating channel for a first signal corresponding to the sum of a pair of stereophonically-related signals, means providing a second signal translating channel for a second signal corresponding to the difference between said pair of stereophonically-related signals, matrix circuit means coupled to said first and second signal translating channels for adding and subtracting the signal from said first and second signal translating channels to provide a pair of output signals, during stereophonic signal reception said pair of output signals corresponding respectively to said pair of stereophonically-related signals and during monophonic reception said pair of output signals both corresponding to the signals in said first signal translating channel, means responsive to said pilot signal for developing a control voltage, and signal translating control means in said second channel coupled to receive said control voltage and responsive thereto to open said second channel for translation of signals therethrough during the reception of stereophonic radio signals including a pilot signal, and operable to block said second channel during the reception of monophonic radio signals when no pilot signal is received.

17. A stereophonic radio signal receiver for stereophonically modulated carrier waves including modulation components corresponding to a pilot signal of fixed frequency, comprising the combination of, means providing a first signal translating channel for a first signal corresponding to the sum of a pair of stereophonically related signals, means providing a second signal translating channel for a second signal corresponding to the difference between said pair of stereophonically-related signals, means coupled to said first and second signal translating channels to provide a pair of output signals, during stereophonic signal reception, said pair of output signals corresponding respectively to said pair of stereophonically related signals and during monophonic signal reception said pair of output signals both corresponding to the signals in said first signal translating channel, means connected in said second signal translating channel operable between a first condition for enabling said second channel to permit signal translation therethrough, and a second condition for disabling said channel to prevent signal translation therethrough, and means responsive to the presence of said pilot signal coupled to cause the means connected in said second signal channel to establish said first condition.

18. A stereophonic sound signal receiver for .a multiplicatively modulated carrier wave which is angle modulated by a first signal wherein the maximum frequency deviation of said carrier wave, for maximum amplitude first signals of frequencies up to the order of said maximum deviation, is a small fraction of the audio signal frequency range to be transmitted and wherein the higher frequency modulating signals are preemphasized by an amount in substantial excess of 75 microseconds, and amplitude modulated by a second stereophonically related signal, said receiver comprising: a common amplifying channel for the reception and amplification of said Wave, means coupled to said common channel and including a detector for detecting` the amplitude modulation of said received wave to reproduce said second signal, means coupled to said common channel and including a frequency modulation detector for recovering the angle modulation component from said received Wave, means for deemphasizing said recovered angle modulation cornponent by an amount in substantial excess of 75 microseconds corresponding to the amount of preemphasis applied to said first signal, utilization means, and means for applying the recovered second signal and the deempha` sized iirst signal to said utilization means.

6* 3 References Cited in the tile of this patent UNITED STATES PATENTS 2,098,561 Beers Nov. 9,1937 2,357,975 Van Roberts Sept. 12, 1944 2,474,244 Grieg June 28, 1949 2,491,918 De Boer et al. Dec. 20, 1949 2,512,530 OBrian et al June 20, 1950 2,532,150 De Boer Nov. 28, 1950 2,617,923l ReKart Nov. 1, 1952 2,698,379 Boelens et al Dec. 28, 1954 2,714,633 Fine Aug.. 2, 1955 2,851,532 Crosby Sept, 9, 1958 2,878,319 Leek Mar.V 12, 1959 2,912,492 Haantjes Nov. 10, 1959 FOREIGN PATENTS 540,185 Great Britain Oct. 8, 1941 OTHER REFERENCES Day: The FM/ Multiplex Converter, Audio, August 1958 (pp. 19-22 and 97-98 relied on). 

1. A STEREOPHONIC SOUND SIGNALLING SYSTEM FOR TRANSMISSION AND RECEPTION WITHIN A GIVEN TRANSMISSION CHANNEL BANDWIDTH, SAID SYSTEM COMPRISING: A SOURCE OF FIRST AND SECOND STEREOPHONICALLY RELATED SIGNALS; A SOURCE OF A CARRIER WAVE SOURCE FOR ANGLE MODULATING AND AMSAID CARRIER WAVE SOURCE FOR ANGLE MODULATING AND AMPLITUDE MODULATING A CARRIER WAVE WITH SAID FIRST AND SECOND SIGNALS, RESPECTIVELY, TO PRODUCE A MULTIPLICATIVELY MODULATED CARRIER WAVE HAVING BOTH ANGLE MODULATION AND AMPLITUDE MODULATION SIDEBANDS WHERE THE MAXIMUM FREQUENCY DEVIATION OF SAID CARRIER WAVE, IN RESPONSE TO MAXIMUM AMPLITUDE MODULATING SIGNALS OF FREQUENCIES UP TO THE ORDER OF SAID MAXIMUM DEVIATION, IS A SMALL FRACTION OF SUCH CHANNEL BANDWIDTH, SAID MEANS FOR ANGLE MODULATING SAID CARRIER WAVE INCLUDING MEANS FOR PREEMPHASIZING THE HIGHER MODULATING FREQUENCIES OF SAID FIRST SIGNAL BY A PREDETERMINED AMOUNT; MEANS FOR TRANSMITTING SAID MULTIPLICATIVELY MODULATED CARRIER WAVE; AND A RECEIVER FOR SAD TRANSMITTED WAVE, SAID RECEIVER COMPRISING A COMMON AMPLIFYING CHANNEL FOR THE RECEPTION AND AMPLIFICATION OF SAID MULITPLICATIVE WAVE, A FIRST STEREO CHANNEL COUPLED TO SAID COMMON CHANNEL AND INCLUDING AN AMPLITUDE MODULATION DETECTOR FRO DETECTING THE AMPLITUDE MODULATION OF SAID RECEIVED WAVE TO REPRODUCE SAID SECOND SIGNAL, A SECOND STEREO CHANNEL COUPLED TO SAID COMMON CHANNEL AND INCLUDING A FREQUENCY MODULATION DETECTOR FOR RECOVERING THE ANGLE MODULATION COMPONENT FROM SAID RECEIVED WAVE AND MEANS FOR DEEMPHASIZING SAID RECOVERED ANGLE MODULATION COMPONENT BY AN AMOUNT CORRESPONDING TO THE AMOUNT OF PREEMPHASIS APPLIED TO SAID FIRST SIGNAL, A SOUND REPRODUCING SYSTEM, AND MEANS FOR APPLYING SAID RECOVERED SECOND SIGNAL AND SAID DEEMPHASIZED FIRST SIGNAL TO SAID SOUND REPRODUCING SYSTEM. 